After years of security briefings and thwarting terrorist plots, national security still keeps Eric Holder awake.

"I still worry at night," the U.S. attorney general told ABC's Pierre Thomas in a wide-ranging, exclusive interview on Wednesday.

"I'm concerned about whether or not we have done all that we can to ensure that every threat has been adequately examined, that we put up our defenses in appropriate ways," Holder said. "So, yeah, I still go to bed worried at night."

America now confronts a very different terrorist threat than it did four years ago, according to Holder. While al Qaeda's central leadership has been diminished, the group is more "widespread" geographically—and the threat of homegrown terrorists in the U.S. now rivals that of plots hatched overseas, Holder told ABC News.

"Core al Qaeda doesn't have the capacity that it once did, but it's metastasized in a number of ways. We now worry about the nodes of al Qaeda—al Qaeda in the Arabian Peninsula, al Qaeda now in Africa, al Qaeda in Iraq," Holder said. "The threat is more widespread in terms of where those people are, where those significant people are. That's how it's different now."

Portions of the interview will air Wednesday, February 27 on "ABC World News"

The nation's top law enforcer worries that Americans could become "complacent" about the threat of terrorism, even as the FBI has thwarted not only international plots, but homegrown terrorists that Holder called "a very serious threat."

"I worry a little that the American people, from the general population, has become a little complacent that we don't understand or realize that the threats are still real, that the danger is out there, is still tangible, that we still have to be as vigilant as we need to be," Holder said.

During his tenure as attorney general, federal authorities have thwarted numerous terrorist plots hatched within the U.S., and Holder told ABC News that the threat of homegrown plots warrants as much attention as international terrorism. In December, the FBI arrested two Florida brothers, Raees Alam and Sheheryar Alam Qazi, alleging they had sought to obtain explosives and carry out a terrorist attack in New York City. The two were charged with conspiracy to use a weapon of mass destruction and conspiracy to provide material support to terrorists.

"It's a very serious threat. I think what it says is that the scope, our scope, has to be broadened. We can't think that it's just a bunch of people in caves in some part of the world," Holder said. "We have to be concerned about the homeland to the same extent that we are worried about the threat coming from overseas."

The FBI's success has made the threat of terrorism less visible: When attempted terrorists are apprehended, the American public doesn't always see or hear much about them, save in cases like the attempted 2009 Christmas bomber, Umar Farouk Abdulmutallab. In his interview with ABC News, Holder warned that thwarted plots shouldn't be dismissed or ignored.

"When those terrorist attempts are thwarted, they are not as scary as a successful attempt, but that doesn't mean that they were any less serious," Holder said. "That's something that the American people need to focus on. When we stop these attacks, that's an indication that the threat is real, it's ongoing, and we have to be very serious about stopping people who continue to want to do harm to the American people."